bash bash中的条件重定向

conditional redirection in bash

提问by daniel kullmann

I have a bash script that I want to be quiet when run without attached tty (like from cron). I now was looking for a way to conditionally redirect output to /dev/null in a single line. This is an example of what I had in mind, but I will have many more commands that do output in the script

我有一个 bash 脚本，我想在没有附加 tty 的情况下运行时保持安静（例如来自 cron）。我现在正在寻找一种在一行中有条件地将输出重定向到 /dev/null 的方法。这是我想到的一个例子，但我会有更多的命令在脚本中输出

#!/bin/bash
# conditional-redirect.sh
if tty -s; then 
  REDIRECT=
else 
  REDIRECT=">& /dev/null"
fi
echo "is this visible?" $REDIRECT

Unfortunately, this does not work:

不幸的是，这不起作用：

$ ./conditional-redirect.sh
is this visible?
$ echo "" | ./conditional-redirect.sh 
is this visible? >& /dev/null

what I don't want to do is duplicate all commands in a with-redirection or with-no-redirection variant:

我不想做的是复制带重定向或不带重定向变体中的所有命令：

if tty -s; then 
  echo "is this visible?"
else 
  echo "is this visible?" >& /dev/null
fi

EDIT:

编辑：

It would be great if the solution would provide me a way to output something in "quiet" mode, e.g. when something is really wrong, I might want to get a notice from cron.

如果该解决方案能为我提供一种以“安静”模式输出某些内容的方法，那就太好了，例如，当某些事情真的出错时，我可能想从 cron 收到通知。

回答by paxdiablo

For bash, you can use the line:

对于bash，您可以使用以下行：

exec &>/dev/null

This will direct allstdoutand stderrto /dev/nullfrom that point on. It uses the non-argument version of exec.

这将直接所有stdout，并stderr以/dev/null从该点。它使用exec.

Normally, something like exec xyzzywould replace the program in the current process with a new program but you can use this non-argument version to simply modify redirections while keeping the current program.

通常，类似的事情exec xyzzy会用新程序替换当前进程中的程序，但您可以使用此无参数版本来简单地修改重定向，同时保留当前程序。

So, in your specific case, you could use something like:

因此，在您的特定情况下，您可以使用以下内容：

tty -s
if [[ $? -eq 1 ]] ; then
    exec &>/dev/null
fi

If you want the majority of output to be discarded but still want to output some stuff, you can create a new file handle to do that. Something like:

如果您希望丢弃大部分输出但仍想输出一些内容，您可以创建一个新的文件句柄来做到这一点。就像是：

tty -s
if [[ $? -eq 1 ]] ; then
  exec 3>&1 &>/dev/null
else 
  exec 3>&1
fi
echo Normal               # won't see this.
echo Failure >&3          # will see this.

回答by daniel kullmann

I found another solution, but I feel it is clumsy, compared to paxdiablo's answer:

我找到了另一个解决方案，但与 paxdiablo 的答案相比，我觉得它很笨拙：

if tty -s; then 
  REDIRECT=/dev/tty
else 
  REDIRECT=/dev/null
fi
echo "Normal output" &> $REDIRECT

回答by Codoscope

You can use a function:

您可以使用一个函数：

function the_code {
    echo "is this visible?"
    # as many code lines as you want
}

if tty -s; then # or other condition
  the_code
else 
  the_code >& /dev/null
fi